FIG. 1 depicts the salient components of an optical telecommunications system in the prior art. The system comprises: optical fiber 110 and optical receiver 100, which itself comprises: photodetector 101, transimpedance amplifier 102, and receiver circuit 103. Optical fiber 110 is used to carry optical signals, and optical receivers are used in optical communications systems to detect and convert optical signals into electrical signals that can be processed by electronic systems or converted into sound.
Photodetector 101 is a device that receives light from optical fiber 110 and outputs an electrical current that is based on the intensity of the light. Transimpedance amplifier 102 converts the current signal from photodetector 101 into a voltage signal for receiver circuit 103. Receiver circuit 206 comprises electronics for processing the input signal in preparation for output to the rest of a processing system.
The quality of the output of the optical receiver is measured by a parameter known as the signal-to-noise ratio, which is the ratio of the magnitude of the output signal to the magnitude of the noise output by the optical receiver. Each component in transimpedance amplifier 102 is a source of some of the noise, and the manner in which transimpedance amplifier 102 is designed affects the quantity and quality of the noise.
FIG. 2 depicts the salient components of transimpedance amplifier 102 in the prior art. Transimpedance amplifier 102 comprises transimpedance stage 204, amplification stage 205, output stage 206, feedback network 207, and converter 208. The input current signal from photodetector 101 is carried on line 109 to one of two inputs of transimpedance stage 204. Transimpedance stage 204 converts the input current signal into a differential voltage signal (i.e. the signal comprises the difference between the voltages carried individually on each of two lines). Amplification stage 205, which provides amplification of the voltage signal (i.e. gain), amplifies the differential voltage in a manner well-known to those skilled in the art. Output stage 206 provides an appropriate interface between the output of amplification stage 205 and receiver circuit 103, in a manner well-known to those skilled in the art.
The differential voltage output signal on output lines 110A and 110B comprises a noise component known as Common-Mode Voltage. This common-mode voltage causes a voltage offset of the signal that degrades the performance of transimpedance amplifier 102. Feedback network 207 is positioned between the differential output of transimpedance amplifier 102 (i.e., the output of output stage 206) and the two inputs of transimpedance stage 204 for the purpose of canceling the voltage offset associated with the common-mode voltage. Converter 208 converts the voltage signals on each of the two output lines of feedback network 207 into current signal inputs for transimpedance stage 204. In some cases, converter 208 is included as part of transimpedance stage 204, and in other cases it is included as part of feedback network 207.
Since feedback network 207 spans the entire topology of the signal path of transimpedance amplifier 102, the total phase shift in the feedback signal is equal to the sum of the phase shift contributed by each of the components in the signal path. The signal-to-noise ratio of transimpedance amplifier 102 is, therefore, degraded by the phase shift associated with the feedback signal.
Transimpedance amplifier 102 is required to provide enough amplification to provide an input signal to receiver circuit 103 with a suitable voltage level. However, too much noise is introduced into the output signal of transimpedance amplifier 102 if transimpedance stage 204 has a transimpedance gain that is too high. These competing requirements lead to a need to include amplification stage 205, and they also determine the magnitude of signal amplification required of amplification stage 205. Similarly, the gain of any one amplifier included in amplification stage 205 should be kept relatively low so as to reduce generated noise on the signal. As a result, in order to obtain sufficient amplification from amplification stage 205 to ensure maximum signal strength at the input of output stage 206, amplification stage 205 comprises a cascade of many limiting amplifiers—each of which introduces some amount of phase shift which further degrades the signal-to-noise ratio of transimpedance amplifier 102.
It is often difficult build a transimpedance amplifier with a lot of gain, but low phase shift, and, therefore, a trade-off exists between gain and phase shift. Furthermore, it is challenging to suppress the voltage offset caused by the common-mode voltage without inducing phase shift.
Therefore, the need exists for an optical receiver that avoids or mitigates some or all of these problems.